Sandbank
It's been a while since I retired from BA but it used to be that if the FO is handling the aircraft (i.e. his sector) he will act as PIC throughout the whole flight (PICUS); Pilot In Command Under Supervision.
The Captain does indeed have every right to assume control of the aeroplane and would do so if needs be. In an incident such as this, it's my guess that it would be mighty tempting for the Captain to take over the controls; however, the ability of the FO is usually known and unless he's making a real mess of things it's best to leave him handling the aeroplane... it takes a significant period to hand-over and adjust yourself to how the aircraft is responding, so it might actually make things worse when within the final stage of the approach to assume control.
This would always be a 'command' decision. Command comes with both training and experience and you cannot always put everything into neat little boxes; therefore, the 'commander' has a huge responsibilty in making those decisions. I believe that in this particular scenario that we will discover that the FO in question isn't that far from achieving his own command; which the Captain would have known and taken into considderation at the time... that's what makes a good commander!
TCF